By Steve Goddard
ESA’s Venus Express mission has been studying the planet and a basic atmospheric model is emerging.
Venus has long been the CO2 bogeyman of climate science. In my last piece about Venus I laid out arguments against the claim that it is a runaway greenhouse which makes Venus hot. This generated a lot of discussion. I’m not going to review that discussion, but instead will pose a few ideas which should make the concepts clear to almost everybody.
If there were no Sun (or other external energy source) atmospheric temperature would approach absolute zero. As a result there would be almost no atmospheric pressure on any planet -> PV = nRT.
Because we have a sun providing energy to the periphery of the atmospheric system, the atmosphere circulates vertically and horizontally to maintain equilibrium. Falling air moves to regions of higher pressure, compresses and warms. The greater the pressure, the greater the warming. Rising air moves to regions of lower pressure, expands, and cools. The amount of warming (or cooling) per unit distance is described as the “lapse rate.” On Earth the dry lapse rate is 9.760 K/km. On Venus, the dry lapse rate is similar at 10.468 K/km. This means that with each km of elevation you gain on either Earth or Venus, the temperature drops by about 10C.
It is very important to note that despite radically different compositions, both atmospheres have approximately the same dry lapse rate. This tells us that the primary factor affecting the temperature is the thickness of the atmosphere, not the composition. Because Venus has a much thicker atmosphere than Earth, the temperature is much higher.
dT = -10 * dh where T is temperature and h is height.
With a constant lapse rate, an atmosphere twice as thick would be twice as warm. Three times as thick would be three times as warm. etc. Now let’s do some experiments using this information.
Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more. Now suppose that we could instantly change the molecular composition of Venus atmosphere to match that of Earth. Because the lapse rate of Earth’s atmosphere is very similar to that of Venus, we would see little change in Venus temperature.
Experiment #2 – Now, lets keep the atmospheric composition of Venus constant, but instead remove almost 91/92 of it – to make the mass and thickness of Venus atmosphere similar to earth. Because lapse rates are similar between the two planets, temperatures would become similar to those on earth.
Experiment #3 – Let’s take Earth’s atmosphere and replace the composition with that of Venus. Because the lapse rates are similar, the temperature on Earth would not change very much.
Experiment #4 – Let’s keep the composition of Earth’s atmosphere fixed, but increase the amount of gas in the atmosphere by 92X. Because the lapse rates are similar, the temperature on Earth would become very hot, like Venus.
Now let’s look at measured data :
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-p.gif
Note that at one Earth atmospheric pressure on Venus (altitude 50km) temperatures are only about 50 degrees warmer than earth temperatures. This is another indication that atmospheric composition is less important than thickness.
Conclusions : It isn’t the large amount of CO2 which makes Venus hot, rather it is the thick atmosphere being continuously heated by external sources. It isn’t the lack of CO2 on Earth which keeps Earth relatively cool, rather it is the thin atmosphere. Mars is even colder than earth despite having a 95% CO2 atmosphere, because it’s atmosphere is very thin. If greenhouse gases were responsible for the high temperatures on Venus (rather than atmospheric thickness) we would mathematically have to see a much higher lapse rate than on Earth – but we don’t.
WUWT commentor Julian Braggins provided a very useful link which adds a lot of important information.
“The much ballyhooed greenhouse effect of Venus’s carbon dioxide atmosphere can account for only part of the heating and evidence for other heating mechanisms is now in a turmoil,” confirmed Richard Kerr in Science magazine in 1980.
The greenhouse theory does not explain the even surface temperatures from the equator to the poles: “atmospheric temperature and pressure in most of the atmosphere (99 percent of it) are almost identical everywhere on Venus – at the equator, at high latitudes, and in both the planet’s day and night hemispheres. This, in turn, means the Venus weather machine is very efficient in distributing heat evenly,” suggested NASA News in April 1979. Firsoff pointed out the fallacy of the last statement: “To say that the vigorous circulation (of the atmosphere) smooths out the temperature differences will not do, for, firstly, if these differences were smoothed out the flow would stop and, secondly, an effect cannot be its own cause. We are thus left with an unresolved contradiction.”
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An update for those interested in what Venus looks like at the surface.
On March 1, 1982, the Soviet Venera 13 lander survived for 127 minutes (the planned design life was 32 minutes) in an environment with a temperature of 457 °C (855 °F) and a pressure of 89 Earth atmospheres (9.0 MPa). The photo composite above shows the soil and rocks near the lander.
Here’s another Venera image that shows a hint of yellow atmosphere. – Anthony
Doc Martyn:
There are clouds because of the atmospheric lapse rate that Steve Goddard talks about.
The lower atmosphere is too hot for certain compounds to condense, but the upper atmosphere (50-70 km altitude) is much cooler, allowing them to condense into clouds. (Example: the pressure at 50km altitude is just below 1 atmosphere and the temperature is 57C.) At the altitudes where these clouds exist, there is very likely a phenomenon of solar energy vaporizing the clouds and heating the atmosphere, the heated gas rises to a higher altitude but cools thanks to the lapse rate, forming new clouds.
Jbar
Your arguments don’t work.
Night on Venus is thousands of hours long, and the temperature is the same as the day side – which doesn’t receive much SW either.
Almost all of the IR is absorbed by Earth’s atmosphere, but we seem to be able to keep temperatures below 733K.
There seem to be two possibilities here:
either
(a) Steven Goddard has made a major, Nobel-worthy breakthrough in atmospheric physics
or
(b) he has fundamentally misunderstood and misapplied the ‘ideal gas law’.
As a scientific layman I am not competent to say which, though, like Quasimodo, I have a hunch.
Jbar says:
May 9, 2010 at 5:00 am
You’re right when the atmosphere blocks IR. I was referring to the supposed GHG-free atmosphere.
Alternatively, it is still true that the surface emits (about) 12,000 W/m2. The S-B law applies at that level. Of course, many wavelengths are than quickly absorbed in the real atmosphere. But they were emitted.
Dave:
May 8 6:48 PM Steve Goddard more-or-less states that there are both lapse rate and greenhouse effect. (Steve:”I’m not sure where anyone got the idea that I am suggesting that there isn’t a greenhouse effect on Venus. It certainly wasn’t from anything I have written. This is a discussion of relative magnitude.”)
So from that I take it his opinion is that it’s just a question of what share of the high surface temperature is due to each effect. However, he doesn’t really say what % is lapse rate and what % is greenhouse. I think none of us in this forum have to tools to calculate that. (Although I’ll take a wild guess its 50/50, plus or minus 25% either way. Does that help???)
I’ve written a post here to try to explain the close relation between the adiabatic lapse rate and the greenhouse effect. It doesn’t make sense to say that warming is due to the lapse rate and not the GHE. They go together.
I have not seen any discussion as to WHY the atmosphere on Venus evolved differently than on Earth. To me that is an important question. If the evolution is different, then there there should be little fear that Earth will end up like Venus.
I understand that Venus has no magnetic field that has allowed the solar wind to strip away lighter elements including water vapor. Add in the slower, retrograde rotation and any comparison to how Earth might evolve starts to become meaningless.
Gareth,
“Convection currents on Earth cause warm air to rise, expand and cool whilst pushing cool air down, compressing and warming it. If Venus has convection currents it is not behaving any differently.”
As I understand it, convection comes from air that is being heated (and hence expands and then rise as it is lighter that the surrounding air) either by the sunlight on the surface or by the sunlight itself or both. If we turn off the energy source the convection stops, which also happens on earth during the night. What this tells us, is that without an external energy source as the sun, the temp on the surface on the earth will drop rapidly due to energy being lost to space.
stevengoddard says:
May 8, 2010 at 10:26 pm
Phil.
You must have forgotten your high school chemistry.
A mole is moist definitely a non-dimensional number. It is 6.023 * 10^23 molecules.
Clearly then it is not dimensionless, as I said it is the unit that represents the amount of a substance.
One of the interesting properties of gases (Avogadro’s law) is that they all occupy about the same volume at the same pressure and temperature, regardless of molecular weight. That is why the ideal gas law does not have a term for mass.
Of course it does!
PV=mRT/M where m is the mass of the gas and M is the molar mass.
Spector:
Let’s do a thought experiment. If a “supernova flash-boils the ocean” as you suggest, then you have an extremely high water vapor concentration in the atmosphere causing a surface pressure of 265 atmospheres (including N2 and O2). At that concentration, water vapor’s capacity to block infrared would turn Earth into a Venus, at least temporarily, and the oceans might never condense again. At that pressure, surface temperature would have to fall below 390C for water to condense.
On the other hand, the whole atmosphere would be enshrouded in water vapor clouds. Some clouds reflect light but others trap infrared, so whether or not this brave new world would stay stuck in “runaway greenhouse” mode or reflect more light back into space and cool back down (allowing the oceans to reform) depends on what type of clouds dominate. Even if the globe cooled enough to form oceans, there might still be enough water vapor left in the atmosphere to keep the surface at some higher pressure and temperature than today (greater than 100C), depending on the balance of cloud types and global albedo.
Either way, the intense blast of radiation from any supernova close enough to boil the oceans would directly kill every living organism on the planet instantly. Game over. Reboot.
Jaymam:
Energy emissions from the Earth’s interior amount to 1/10th of a watt/m2, thousands of times less than the surface radiation from the sun. This energy source is only significant to the behavior of thousands of miles of rock with a low heat transport rate, not the atmosphere or oceans with their much higher heat transport rates.
stevengoddard says:
May 8, 2010 at 5:53 pm
I’m not sure what people are arguing about at this point, but let me try to set some basics right. Much of physics can be reduced to the basic units of mass, length, and time. Before the term “SI” replaced “metric”, i.e. back in my high school physics class, we talked about measuring things in CGS (centimeter, gram, second) vs. MKS (meter, kilogram, second) units. Maybe also FPS (foot, poundal, second) but let’s not go there. I’m not sure why we didn’t include temperature, probably because we were looking at static and dynamics (hanging signs, weights on massless ropes a pulleys and baseballs thrown on flat, airless planets).
So, the ideal gas law is
pV = nRT
From Wikipedia (which is a good source for some stuff like this), In SI units, p is measured in pascals; V in cubic metres; n in moles; and T in kelvins. R has the value 8.314472 J·K^-1·mol^-1.
A pascal is pressure, and is newton per meter squared. A newton is force, and its units come from F=ma, so mass times distance per time squared, or g·m·s^-2.
Therefore, a pascal is g·m^-1·s^-2 due to dividing be square meters.
Volume is m^3, n is dimensionless (a number of molecules, but granted what I call a “honorary unit” of mol), temperature just K.
R has to make the dimensions balance. Wikipedia uses Joules in its J·K^-1·mol^-1. A joule is a unit of energy, which is force times a distance (parallel to the direction of the force!). We have force above, so a joule’s dimensions are g·m^2·s^-2.
Does all this balance? pV is g·m^-1·s^-2 · m^3 = g·m^2·s^-2
nRT is g·m^2·s^-2·K^-1·mol^-1 · mol · K = g·m^2·s^-2
Yay – it balances. Mass is a basic unit in pressure and energy.
However, mass can be ignored for looking at small quantities of gas as it is compressed and expanded adiabatically. It cannot be ignored when looking at tall columns of a gas, e.g. a column of an atmosphere, because the mass above any point in the column determines the pressure at that point.
Assuming a well mixed column, as we seem to have at Venus thanks to surface heat causing convection to mix the troposphere, then given the temperature at any point in the column, we can determine the temperature when a handful of gas there is moved anywhere else in the column.
jcrabb
Most of the action in Venus’s atmosphere takes place in and above the cloud layer (50-70km altitude). Here clouds absorb sunlight on the sun side and race around at high speed to emit infrared radiation on the night side, keeping the temperature of this high atmospheric layer relatively uniform in spite of the slow rotation of the surface. The uniform temperature of this high altitude zone helps to set the uniform temperature distribution of the lower atmosphere and surface. (Not sure of the details of that.)
Venus’s atmosphere is highly refractive, so that sunlight wraps around the planet from the sun side all the way to the dark side, much much more than the dawn/dusk effect on Earth (although I have no idea how much sunlight actually reaches the dark side). That provides some solar heating of the surface even on the dark side. However, I am still at a loss to explain why the surface temperature is uniform all the way around unless the high refractivity of the atmosphere makes the surface lighting uniform. (Doubtful)
pft
The ocean does not freeze at the bottom because water is most dense a few degrees above freezing. It cools at the surface of the ocean and then sinks down before it freezes. It cannot freeze then because there is no mechanism to cool it to a lower temperature down there.
We’re very lucky that water works this way or ice would continue to pile up on the ocean floor until all the oceans and lakes would be entirely frozen over and our planet would be an ice ball.
stevengoddard says:
May 8, 2010 at 5:53 pm
Degrees Kelvin is a unit of temperature
<rant>
There are degrees Celcius, there are degrees Fahrenheit, there are no such things as degrees Kelvin. The term is just kelvins, lowercase! Otherwise, for consistency’s sake you’d want to talk about length meters (aren’t those rulers?), time seconds (which almost makes sense to distinguish from angle seconds), and frequency hertz (units named for people are always lower case).
If we’re going to get the basics right, lets start with the names.
</rant>
Jbar says:
May 9, 2010 at 5:00 am
Nick Stokes,
The 700K surface of Venus doesn’t emit 12,000 W/m2. That would be (close to) the emittance of a black body radiating at near 700K, but Venus’s surface is NOT a black body.
Venus’s surface must be 733K to remain in radiative balance with sunlight because almost the entire infrared spectrum is blocked by the CO2 atmosphere and water vapor at high altitude. When you block parts of the infrared spectrum, the Stefan Boltzmann law no longer applies. Instead of having a simple formula based on T^4, now you have to integrate over the whole emission spectrum wavelength by wavelength, excluding the wavelengths that are blocked.
Whether the atmosphere is blocked has nothing to do with the emissivity of the surface, which remains a near black body.
A “mole” is shorthand for chemists to make it easier for them to do their maths.
It is a number of grams equivalent in value to the molecule’s molecular weight. Eg. one gram-mole of water = 18 grams of water (approx), same as the molecular weight of water.
A mole of any substance contains the same number of molecules – Avogadro’s number – 6.022 E23 molecules.
So if a chemist knows that one molecule of oxygen (O2) reacts with two molecules of hydrogen (H2), they know that they need one mole of oxygen (16 grams) to react with 2 moles of hydrogen (2 grams) to make 1 mole of water (18 grams).
It is much more convenient than counting molecules!
Engineers play with bigger numbers than chemists so US and British engineers like to use “pound-moles”, or the number of pounds with the same numeric value as the molecular weigh.
Oh – I forgot to mention a couple posts back about my “honorary” units. I’ve found them handy in some calculations and in “dimensional analysis” to give units to technically dimensionless quantities. The simplest example is pi, 3.14159….
Pi is dimensionless, so the equation circumference = 2 x pi x radius is just length on both sides. If we use the honorary units of radial meters and circumferential meters, then pi takes on units of circumference/radius and becomes easier to track. You can also thrown in units for diameter and radius lengths to keep the “2” involved in the dimensional analysis.
Dimensional analysis, by the way, is something I learned quickly in physics class, but isn’t taught very well. I remember reading an article in Science News decades ago discussing that and claiming that students taught it understood physics much better and applied various formulae correctly.
One thing I never did learn, and was in part my downfall in college physics, is the Greek alphabet. Both upper and lower case. Reading equations like “P = squiggle times squiggle-with-extra-zig divided by lambda” was not the way to learn that stuff. At least I know lambda, well, lowercase lambda anyway.
The amount of energy coming out of the earth is 4.42E+13 watts
The amount of energy reaching the top of the atmosphere from the sun is 1366 watts/m2
The average albedo of the Earth is about 0.3 therefore 410 watts/m2 is not reflected
Area of Earth’s surface 510,072,000 km2 = 5.1E+14 m2
Area of Earth with the sun shining on it is 4 times less, i.e. 1.275E+14 m2
Energy from the sun is 1.275E+14 * 410 = 5.22E+16 watts
5.22E+16 divided by 4.42E+13 = 1182, or 0.085%
0.085% should not be ignored.
jcrabb says:
May 9, 2010 at 12:22 am
As day and night temperatures are so similar on Venus something must be maintaining the night time temperatures, especially considering the Venusian night is around 120 Earth days long, surely without the direct input of heat from the Sun over this period a significant amount of heat would be lost, unless there was a significant Greenhouse effect at work.
The relatively small difference between day and night temps intrigues me as well. Wouldn’t a reasonable assumption be that the thick atmosphere is circulating so fast that it is distributing energy around the planet so that convective heat distribution is dominant?
Dave McKay
The 380 ppm of CO2 in our atmosphere absorbs ALL the infrared radiation between 13 and 18 microns wavelength trying to leave the surface of the Earth on its way to space. The CO2 then reradiates that energy in all directions, up, down, and sideways. (Anything going sideways has to travel through so much more atmosphere than what’s going straight up that it effectively winds up getting all reabsorbed, so the net effect is for the radiation to either pass up or be reflected effectively straight down.) So the CO2 acts like a few-percent reflective mirror of infrared surrounding the Earth.
The atmosphere of Venus contains 89,000,000 ppm (relative to Earth), which is enough to block all the infrared radiation between 2 and 28 microns wavelength except for 3 narrow windows of transparency and everything longer than 28 microns. So even a few kilometers of Venus’s CO2 atmosphere absorbs the majority of radiation trying to leave the surface and reradiates it in all directions (but, effectively, 50-some% up/ 40-some% down). Then the next few km higher does it again, and the next few km, and the next, until the radiation finally gets to the top of the atmosphere and can get out freely into space. So the CO2 in Venus’s atmosphere acts like a tall stack of 40-50%-reflective infrared mirrors.
There’s water vapor high up in Venus’s atmosphere, and that water is enough to block all the infrared radiation longer than 28 microns, the big chunk that CO2 misses, adding H20-greenhouse insult to CO2-greenhouse injury.
If the seas of Venus boiled in a runaway greenhouse meltdown, where is the water?
A different approach to Venus:
https://ssl.scroogle.org/cgi-bin/nbbwssl.cgi
Without greenhouse gases in Venus’s atmosphere, Venus would be hundreds of degrees cooler. That means Venus is that hot because of the greenhouse effect.
Talk about pressures and lapse rates is just dancing around the above fact.
Steve Goddard
I guess all those moles or co2 is causing all your cold weather in CO.